One of the primary goals of lubricants is obtaining low friction. Solid lubrication offers many benefits over conventional oil-based hydrodynamic and boundary lubrication. Solid lubrication systems are generally more compact and less costly than oil lubricated systems since pumps, lines, filters and reservoirs are usually required in oil lubricated systems. Greases can contaminate the product of the system being lubricated, making it undesirable for food processing, and grease and oil outgas in a vacuum preclude their use in space applications.
In some lubrication applications, sliding electrical contacts connect two electrically conductive members which transmit high current density from one conductive member to the other conductive member across the sliding contact. In such applications, the lubricant material typically must be highly electrically conductive. These applications include a wide variety of military hardware, including slip-rings in tilt wing aircraft, antennae, radar pointing systems, and electrical motors. Conventional solid lubricants currently available generally provide insufficient wear protection for some important applications. For example, even with the use of available solid lubricants to reduce wear rates and friction, current efforts to develop a Superconducting Homopolar Motor (SCHPM) for ship propulsion have been hampered by excessive wear in the brush system which conducts high electrical currents from the rotor to the stator.
Since the brushes of SCHPMs are known to wear during use, designers typically must base design decisions on an assumed wear rate for the brushes based on past experience and projected technology development, which generally results in a minimum required wear length for the brushes. Wear rate is a strong function of contact force. In order to be able to achieve the required wear rates, contact forces need to be kept very low, on the order of about 3 to 4 N. Unfortunately, for electrical contacts between bulk solids, low contact forces can result in high contact resistance as well as excessive heating and losses at the interface. The difficulty is sometimes addressed by the use of multifilament wire brushes, in an attempt to achieve satisfactory contact resistance at these low forces. However, when multifilament brushes are used in the high magnetic fields and high current densities of the SCHPM, electromagnetic forces on the individual filaments of the brush are typically high enough to distort the filaments, often quite significantly, thus changing the true contact force and altering the brush wear rate in some sections of the motor.